1. Field of the Invention
This invention relates to airbag systems and, more particularly to an optical sensor for automobile “smart” airbag systems.
2. Description of the Related Art
Airbags are fitted on practically all new cars. Airbags are devices that are currently acknowledged as being effective in reducing road accident mortality for both drivers and passengers.
While performing this safety function, however, it has been reported that airbags can sometimes injure users.
Typically, an airbag inflates in approximately 0.06 seconds, at an inflation speed which can also be in the order of 320 km/h (200 mph). This rapidity in deployment is essential, since a typical car crash lasts for approximately 0.125 seconds. The vehicle occupants who are not protected can collide with the structures of the passenger compartment before the collision comes to an end. In other words, this second crash or collision occurs while the first crash or collision is still in progress.
Usually, airbags are deployed by means of a gas generator that produces the volume of gas required by means of a rapid combustion of a compound (usually sodium based). This combustion is extremely fast and can, in actual fact, be compared to a small, contained explosion.
Airbag deployment is usually controlled by a processor which, according to the signals received from respective sensors, determines whether to deploy the airbag considering the severity of the accident or not. This occurs on the basis of algorithms that account for the change in vehicle speed and, in the most recent systems, on other factors related to the conditions in which the rapid vehicle deceleration occurs.
The sensors used as usually linear accelerometers, which number and characteristics are adapted according to factors such as the position of the airbag and its function (e.g., front airbag, side airbag, etc.).
As mentioned above, there are situations in which the airbags can cause negative effects. As experience in the use of airbags is being accumulated, the knowledge of such negative effects, which in some cases can have very serious consequences, is being developed.
Usually, the most negative effects are found in relation to individuals who are very close to the airbag when it is deployed. This situation can occur, particularly, to short occupants, children and, in general, to passenger compartment occupants who are either not wearing seat belts or wearing seat belts incorrectly or setting in relatively unusual positions when airbag expansion starts.
These situations can involve, for example, drivers who sit very close to the steering wheel or infants travelling in restraint systems fitted on the front seats, also facing backwards.
The need to improve the functional characteristics of airbags to ensure the protection of all occupants, regardless of their height or build, or whether they are wearing seat belts or not, has also been officially expressed by organizations such as the NHTSA, National Highway Traffic Safety Administration, in the USA. This need particularly refers to the risks to which children, specifically infants, are exposed.
In this perspective, newly designed airbag systems (currently called “smart” airbags) are being developed. These systems, for example, use a weight sensor to trigger the airbag only when a corresponding minimum weight is detected in the corresponding seat. This solution also avoids possible useless deployment of the airbag located in the position corresponding to the seat destined to be occupied by the passenger sitting next to the vehicle driver when this seat is empty.
For example, CTS Corporation has recently developed a number of weight sensors based on film resistors capable of being used in the context of these airbag systems.
There are other systems which, in order to reduce the risks to which the occupants who are too close to the airbag when it is deployed are exposed, regulate the deployment speed of the airbag according to the severity of the collision, particularly by detecting the position of the person immediately before starting airbag deployment or by reducing the deployment force if the person risks being injured.
Usually, the operation of these systems is based on the integration of data from various sensors, such as weight sensors, roll-over sensors, seat belt sensors, side crash sensors and crash intensity sensors, etc. The characteristics of the collision and the position of the occupants determine which airbags to trigger. There are also more complex systems that also control the restraint system (in terms of energy and time) according to the data received from the sensors.
Various different solutions have been presented in patent literature as specifically concerned the detection of the position of occupants in the vehicle passenger cabin.
For example, U.S. Pat. No. 5,330,226 describes a system comprising a shift sensor for signalling the distance between the position of the airbag and the person. For this purpose, an infrared sensor is fitted in the upper part of the passenger cabin over the person.
Patent U.S. Pat. No. 5,528,698 describes a system with image sensor and processor. The sensor is a photo detector array sensor with lens assembly and the processor capable of processing the image thus obtained to acquire the occupancy status of the passenger seat area in the passenger cabin.
In U.S. Pat. No. 5,612,876 a system is described for detecting occupancy in the passenger compartment, particularly to inhibit the deployment of the airbag when no occupant is present. The respective sensor presents a front sensing region and a rear sensing region, which can be evaluated separately.
Moreover, WO-A-94/22693 describes a position sensor employing ultrasound, microwave or optical technology, so to ensure a long sensing time before the occupant crashes into the airbag.
Finally, Ep-A-0 781 589 describes the use of a crash sensor adapted to provide a first signal, a retention system comprising an optical image sensor destined to generate a second signal and a controller, which provides a third signal and an activation system.